A loudspeaker reproduction apparatus installed in the passenger compartment of a vehicle typically has a four-loudspeaker system in which a pair of front loudspeakers are located at both ends of an instrument panel or on the right and a left front doors and a pair of rear loudspeakers are located at both the ends of a rear deck or baggage space. The four-loudspeaker system is provided with a power fader for controlling and balancing the volume of sound from the front and the rear loudspeakers.
FIG. 5 shows a block diagram of a conventional loudspeaker system of the kind just described. The system comprises input terminals 1R and 1L for audio signals in a right and a left channel, respectively, variable resistors 2 and 3 which are operated in conjunction with each other, the sliding terminals 2' and 3' of the variable resistors, power amplifiers 4, 5, 6 and 7, a loudspeaker 8L located in a left front position, a loudspeaker 8R located in a right front position, a loudspeaker 9R located in a left rear position, a loudspeaker 9R located in a right rear position, and resistors R.sub.1, R.sub.2, R.sub.3 and R.sub.4. The audio signal in the left channel of an audio source is sent to the input terminal 1L so that the audio signal is applied to the loudspeakers 8L and 9L, through the resistors R.sub.1 and R.sub.2 and the power amplifiers 4 and 6, and is grounded through the sliding terminal 2' of the variable resistor 2. The audio signal in the right channel is sent to the input terminal 1R so that the audio signal is applied to the loudspeakers 8R and 9R, through the resistors R.sub.3 and R.sub.4 and the power amplifiers 5 and 7, and is grounded through the sliding terminal 3' of the variable resistor 3.
The variable resistors 2 and 3 constitute a power fader which is a front and rear sound volume balance control circuit. The sliding terminals 2' and 3' of the variable resistors 2 and 3 are moveable in order to permit control of the sound volume from the loudspeakers 8R, 8L, 9R and 9L and to permit the volumes to be balanced. When the sliding terminals 2' and 3' are moved in a direction as shown by arrows R in FIG. 5, the outputs from the rear loudspeakers 9R and 9L are increased and those from the front loudspeakers 8R and 8L are decreased. When the sliding terminals 2' and 3' are moved in a direction as shown by arrows F in FIG. 5, the outputs from the front loudspeakers 8R and 8L are increased and those from the rear loudspeakers 9R and 9L are decreased.
Since the size of a rear space in which the rear loudspeakers 9R and 9L are installed often is relatively large, when compared to the available space on an instrument panel, the diameters of the rear loudspeakers can be set at relatively large values of about 6 to 20 cm. Moreover, the trunk room of a vehicle is adjacent to the rear baggage space and can be used as a back cavity for the rear loudspeakers 9R and 9L. Accordingly, the low pitch range of sounds can be reproduced quite well by the rear loudspeakers. On the other hand, the diameters of the front loudspeakers 8R and 8L can be set only at small values of about 10 cm or less because of spatial restrictions. Furthermore, the limited space precludes use of a baffle effect for the front loudspeakers 8R and 8L. For these reasons, it is difficult to adequately reproduce the low pitch range of sounds with the front loudspeakers 8R and 8L.
Only the levels of the audio signals which are to be applied to the loudspeakers 8R, 8L, 9R and 9L are regulated by the conventional power fader. That regulation does not take into consideration the difference between the low-pitch sound reproduction characteristics of the front and the rear loudspeakers. Accordingly, the power fader has a drawback in that the frequency balance between reproduced sounds is subject to collapse. In particular, when the sliding terminals 2' and 3' of the variable resistors 2 and 3 are moved in the direction as shown by the arrows F in FIG. 5, the audio signals are only slightly applied to the rear loudspeakers 9R and 9L. As a result, the reproduced sounds do not adequately reproduce the low pitch range. In other words, the conventional power fader functions to apply the audio signals to the front and the rear loudspeakers in such a manner that the frequency characteristics of the audio signals are not taken into consideration. For that reason, if some of the loudspeakers have good low-pitch sound reproduction characteristics and the others have poor low-pitch sound reproduction characteristics, the quality of the reproduced sound as a whole is significantly changed as the power fader is operated.
The present invention is intended to eliminate the above-mentioned drawback.
Accordingly, it is an object of the present invention to provide a power fader in which a variable resistor is controlled so that even when an audio signal at a high level is applied to a front loudspeaker that has poor low-pitch sound reproduction characteristics, for example, the audio signal in the low pitch range of sounds is reproduced by a rear loudspeaker that has good low-pitch sound reproduction characteristics. Thus, the rear speaker acts maintains the quantity of the low-pitch part of the entire reproduced sound and, thus, maintains the quality of the entire reproduced sound within the listening area.